Question

A car travels 30km at uniform speed of 40km/hr and the next 30km at a uniform speed of 20km/hr . Find the average speed

Answer 1

t1=30/40=3/4h
t2=30/20=3/2h
average speed=30+40/3/4+3/2=280/9km/h

Answer 2

Answer:

80/3 km/hr

Explanation:

Given :

Total distance (d) 30km + 30km = 60km

Speed for first 30 km (u1) = 40 km/hr

Speed for next 30 km (u2) = 20 km/hr

To find :

Average speed = ?

Formula Used :

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \implies \sf \: speed = \dfrac{total \: distance(d)}{total \: time \: taken(t)}$

but , we don't know total time. So , we will find it , by using :-

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \sf \implies \: time = \dfrac{distance}{speed}$

Time in first part of journey :

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \sf \implies \: time \: = \: \dfrac{30}{40} hrs$

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \sf \implies \: time = \dfrac{30}{40} \times 60 = 45 \: minutes.$

Time in second part of the journey :-

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \sf \implies time = \dfrac{30}{20} hrs$

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \sf \implies \: time = \dfrac{30}{20} \times 60 = 90 \: minutes$

Total time = T1 + T2

Total Time = 45+90 = 135 minutes

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \sf \implies \:average \: speed = \dfrac{60 \times 60}{135}$

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \sf \implies \:average \: speed = \frac{80}{3} km \: per \: hour$

Extra points to remember for chapter Motion!!

• Displacement is the distance in a particular direction. A vector quantity.

• Speed is the rate of change of distance. s = D/T